text: Micrometeorite (MM) textures vary as their original structural and mineral compositions are modified by the degree of heating that they experience entering the atmosphere—a function of their initial speed and angle of entry. They range from unmelted particles that retain their original mineralogy (Fig. 1 a, b), to partially melted particles (Fig. 1 c, d) to round melted cosmic spherules (Fig. 1 e, f, g, h, Fig. 2) some of which have lost a large portion of their mass through vaporization (Fig. 1 i). Classification is based on composition and degree of heating.
  The extraterrestrial origins of micrometeorites are determined by microanalyses that show that:
  *The metal they contain is similar to that found in meteorites.
  *Some have wüstite, a high-temperature iron oxide found in meteorite fusion crusts.
  *Their silicate minerals have major and trace elements ratios similar to those in meteorites.
  *The abundances of cosmogenic manganese (Mn) in iron spherules and of cosmogenic beryllium (Be), aluminum (Al), and solar neon isotope in stony MMs are extraterrestrial
  *The presence of pre-solar grains in some MMs and deuterium excesses in ultra-carbonaceous MMs indicates that they are not only extraterrestrial but that some of their components formed before the Solar System.
  An estimated 40,000 ± 20,000 tonnes per year (t/yr) of cosmic dust enters the upper atmosphere each year of which less than 10% (2700 ± 1400 t/yr) is estimated to reach the surface as particles. Therefore the mass of micrometeorites deposited is roughly 50 times higher than that estimated for meteorites, which represent approximately 50 t/yr, and the huge number of particles entering the atmosphere each year (~10 > 10 µm) suggests that large MM collections contain particles from all dust-producing objects in the Solar System including asteroids, comets, and fragments from the Moon and Mars. Large MM collections provide information on the size, composition, atmospheric heating effects and types of materials accreting on Earth while detailed studies of individual MMs give insights into their origin, the nature of the carbon, amino acids and pre-solar grains they contain.
  Chemical analysis of the microscopic chromite crystals, or chrome-spinels, retrieved from micrometeorites in acid baths has shown that primitive achondrites, which represent less than half a percent of the MM reaching Earth today, were common among MMs accreting more than 466 million years ago.